Extracellular glutathione peroxidase (Gpx3) is produced primarily by the proximal convoluted tubule (PCT) cells of the kidney. Gpx3 catabolizes H2O2 and other hydroperoxides outside cells using GSH as its reducing substrate. Thus, Gpx3 functions as an extracellular antioxidant. We have developed immunocytochemistry methods to detect Gpx3 and have used them to show that Gpx3 binds specifically to the basement membranes of kidney tubules, small intestine, colon, and other absorptive epithelia. It does not bind to basement membranes in most tissues. We have also acquired mice with the Gpx3 gene deleted. Using these mice, we have found that extracellular glutathione metabolism is sharply affected by Gpx3 deletion with plasma glutathione concentration being almost tripled. This finding supports GSH as being the reducing substrate of Gpx3. We have also shown that Gpx3 is not involved in the regulated transport of selenium. We hypothesize that Gpx3 protects basement membranes and their associated epithelial cells in kidney and intestine from damage by oxidative stresses. We propose to characterize the relationship of Gpx3 with basement membranes and to assess its effect on oxidative stresses in intestine, kidney, and liver. We also propose to characterize the effect of the kidney on the presence and function of Gpx3 in other tissues. These studies are designed to provide insight into Gpx3 function and to assess how kidney disease might deprive other tissues of Gpx3 and thereby render them susceptible to injury from oxidative stress. PUBLIC HEALTH RELEVANCE: Extracellular glutathione peroxidase (Gpx3) is an antioxidant enzyme. Our preliminary data show that Gpx3 binds to basement membranes in the intestine and in other tissues. We produced mice with Gpx3 deleted and will assess Gpx3 protection against oxidative stress in those mice to shed light on some diseases.